In recent technology of calibrating pressure sensors with a large area, a pressing unit having a rigid structure is often used to directly press on the sensor having a plurality of sensing units, wherein the uniformity of the pressing force of the pressing unit depends on the planarity of the contacting area between the pressing unit and the pressure sensor. As for the rigid structure, the larger the contacting area, it is more difficult to obtain a good planarity, i.e., the worse the planarity would become.
Accordingly, the pressing unit composed of rigid structure needs a very high fabricating cost to have a good planarity, otherwise it would lead to a bad uniformity of pressure due to the bad planarity during pressing process. Both of the ways mentioned above cannot effectively solve the issue of calibrating pressure sensors having a large area.